The actinomycetes include the pathogenic nocardiae, the antibiotic-producing streptomycetes and many microorganisms able to carry out chemical transformation of medically important sterols and able to utilize hydrocarbons. Syncytic recombination, that is, the transfer of genetic information from one prokaryotic organism to another, has been established for many streptomycetes and a few nocardiae. The overall objectives of this project are to describe the genetic structure, recombinational mechanisms and regulatory process of the nocardiae and streptomycetes. The research may be subdivided into four aspects: the control of metabolic activity in Nocardia erythropolis, the regulation of sporulation in Streptomyces venezuelae, characterization of the structure and organization of the genomes of the nocardiae and streptomycetes and an analysis of the factors restricting recombination in the actinomycetes. The results to date indicate that certain catabolic enzymes and permeases are inducible in N. erythropolis; for example, glucose-6-phosphate dehydrogenase, mannitol dehydrogenase and the transport system for succinate. Moreover, the pH-indicating sporulation pigment isolated from S. venezuelae S13 is able to induce sporulation in asporogenous nonpigmenting variants. In addition, the lack of reciprocity in DNA association assays involving selected strains of N. erythropolis indicates that the DNA (genome) from particular lineages possesses unique nucleotide sequences.